| Several commercial alumino-silicate minerals doped with small amounts of light rare-earth oxides were tested for resistance to molten aluminum alloy attack. Although undoped compositions were severely attacked by aluminum alloy 7075, the addition of rare-earth oxides up to 15 percent by weight virtually eliminated the alloy attack. These rare-earth containing compositions were also resistant to the attack of Na{dollar}sb2{dollar}CO{dollar}sb3{dollar} containing alloy. The dopants CeO{dollar}sb2{dollar}, La{dollar}sb2{dollar}O{dollar}sb3{dollar} and bastnasite, a rare-earth concentrate, were all effective in rendering the alumino-silicate minerals resistant to alloy attack.; The addition of rare-earth oxide to bauxite was shown to alter its microstructure, forming a network of uniformly shaped corundum crystals interlinked with cerium containing strands. This unique microstructure was found to be primarily responsible for the enhanced attack resistance of the doped alumino-silicates. Several other possible mechanisms of protection were not supported by the experimental findings. Among these were enhanced sintering, the excess oxygen content of CeO{dollar}sb2{dollar} dopant and higher thermodynamic stability of the doped refractory compositions.; The lattice energies of crystals were calculated from a computer summation of the Coulombic interaction energies. After evaluating this technique using several simple crystal systems, the lattice energies of the light rare-earth (La through Eu) pyrosilicates, RE{dollar}sb2{dollar}Si{dollar}sb2{dollar}O{dollar}sb7{dollar}, were calculated. The lattice energies calculated for simple crystal systems were within 10% of the experimentally measured values. The enthalpies of formation for the rare-earth pyrosilicate were estimated from the calculated lattice energies by applying the Born-Haber cycle. |
